1. Field of the Invention
This invention relates generally to aircraft landing gear braking systems, and more particularly concerns an improved system for protection against inadvertent braking, and multiply redundant separated brake control channels.
2. Description of Related Art
Automatic braking systems have been commonly provided on commercial aircraft to aid the deceleration of the aircraft upon landing. As the size and complexity of aircraft have increased, the automatic braking systems have also become more complex and computerized. Modern anti-skid systems incorporated into aircraft braking systems commonly optimize braking efficiency by adapting to runway conditions and other factors which affect braking in order to optimize deceleration, typically corresponding to the level of brake pressure selected by the pilot.
A catastrophic failure mode can occur in a conventional brake-by-wire control system that results in uncommanded brake application on one or more wheels during takeoff of the aircraft. Since uncommanded braking during takeoff can have serious consequences, and at the very least can result in unnecessary and accelerated wear to the braking system, it is desirable to configure the braking system to reduce the possibility of these undesirable results. The overriding primary consideration is, of course, safety, although considerations of reliability are also significant.
High performance digital brake-by-wire control systems have been developed and installed on several aircraft including light commercial jet transports and modern business jets that use brake pressure feedback and enhanced built-in-test capability. Brake torque control is also used to further enhance brake control. Such digital brake control systems have achieved excellent braking performance over all runway conditions, and in RTO (Refused Take Off) and landing configurations. With sophisticated brake control algorithms, optimum braking performance is assured regardless of conditions, and the same software configuration can be used with a wide range of brake and hydraulic configurations.
Two specific catastrophic failure modes that need to be addressed by an aircraft braking control system architecture are: a) the inadvertent application of any brake during the takeoff roll, and b) the complete loss of braking. The problem of inadvertent application of any brake during the takeoff roll sets the following design requirements: 1) no single failure shall result in the application of any brake during take off; and 2) the probability of any combination of failures leading to any brake being applied during take off shall be extremely improbable (less than 1xc3x9710xe2x88x929). The second catastrophic hazard, the loss of all braking, sets the following design requirements: 1) no single failure shall lead to loss of all braking; and 2) the probability of any combination of failures leading to loss of all braking shall be extremely improbable (less than 1xc3x9710xe2x88x929). These high performance requirements preclude the exclusive use of software. In addition, another commonly known braking control architecture has the disadvantage that the active brake control hydraulic fluid channels are connected to a single coil within the brake control valve, which provides a single point of failure that can result in catastrophic failure in the event of failure at that point.
Redundancy is typically achieved by use of a master or monitor channel that is used to monitor the operational status of hydraulic fluid braking control channels, and the monitor channel can command a first control channel to turn off and a second control channel to commence control, for example. Another method of redundancy management uses two control channels with one active control channel, and a second, inactive control channel in standby mode. When the active channel shuts down, the standby channel takes over control. However, both the master-slave and the active-standby systems can permit a single failure within the master or the active channel to cause a major breakdown in the redundancy management system.
In addition, loss of braking can also occur as a result of the antiskid function, requiring accounting for the probability that normal, alternate, emergency, and ultimate brake systems will be depressurised by a single failure of the anti-skid system. Loss of braking can occur owing to incorrect antiskid activity as a result of control system failure or loss of aircraft power. Another significant failure is the loss of gear retraction braking, which could allow a wheel with a loose tire tread to enter the wheel well while spinning. The hardwired interlock used to prevent application of brakes during take-off, typically conflicts with the requirement to stop the wheels during climb when the thrust levers are advanced.
Furthermore, the need to preclude asymmetric braking as a result of the loss of braking, or extra braking on one main landing gear set the following design requirements: 1) combinations of failures leading to the loss of all braking on either main landing gear shall be improbable (1xc3x9710xe2x88x926); and 2) combinations of failures leading to extra braking on either main landing gear shall be improbable (1xc3x9710xe2x88x926). Touchdown and aquaplaning protection is provided by comparing wheel speeds with the groundspeed signal from the Air Data Inertial Reference Units (ADIRU). Typically any main gear aft wheel that is at a velocity 50 knots or more below the ADIRU groundspeed value is given a brake release signal. Undesired asymmetrical release of brakes can result from a false ADIRU signal, or from unwanted pressure being applied to any brake.
A need therefore continues to exist for an improved aircraft landing gear braking control system. The present invention addresses these and other needs.
Briefly, and in general terms, the present invention provides for a braking control system for dual redundant control of hydraulically operated wheel brakes of aircraft landing gear providing protection against inadvertent braking, and separation of braking control through primary and secondary braking control channels using an interface with dual coil brake control valves. The braking control system is safe, reliable, maintainable, lightweight, and affordable, and provides for a redundant brake-by-wire control architecture using a primary dual redundant brake-by-wire braking system, and a secondary dual redundant analog brake-by-wire system. Positive hydraulic system selection between the normal primary and alternate secondary hydraulic braking systems is performed using solenoid operated shutoff valves (SOSV). The primary braking system control of a center landing gear, if one is present, is split between right and left pedals, with the front axle of center gear landing controlled by left pedals, and the aft axle of center landing gear controlled by right pedals, and locked wheel protection is performed on a tandem basis rather than on an axle basis to prevent fault propagation. Alternate braking is performed on a paired wheel basis through the alternate hydraulic system. The primary braking system includes pressure and antiskid control performed using dual coil servo valves with pressure feedback, autobrake control employing primary brake system servo valves, and an equal load distribution provided by pressure feedback control. Emergency braking is also provided, allowing braking when all electrical power generation and all hydraulic power generation is lost. Parking brake and ultimate braking modes are also provided, using hydraulic power stored in accumulators.
The present invention accordingly provides for a braking control system for dual redundant control of hydraulically operated wheel braking for an aircraft having landing gear that can move between a retracted position and an actuated position, the landing gear having a plurality of wheels and a corresponding plurality of wheel brakes for said plurality of wheels, and a plurality of brake pedals for controlling operation of braking of said plurality of said wheels. In a presently preferred embodiment, a primary hydraulic system is connected in fluid communication with the plurality of wheel brakes for providing hydraulic power for normal operation of the plurality of wheel brakes in a normal braking mode, and a secondary hydraulic system is connected in fluid communication with the plurality of wheel brakes for providing hydraulic power for alternate operation of the plurality of wheel brakes in an alternate braking mode. A control unit is provided for controlling brake pressure communicated to the wheel brakes through the primary and secondary hydraulic systems, and a monitor channel is operatively connected to the primary hydraulic system for detecting faults in the primary and secondary hydraulic systems and for selecting between the primary and secondary hydraulic systems for providing braking pressure. In a presently preferred aspect, the monitor channel detects occurrence of loss of pressure in the primary hydraulic system, if any brake has unwanted pressure applied, and if a fault is detected on the primary or secondary channels that affects more than one wheel brake on each landing gear.
In a presently preferred aspect of the invention, the primary hydraulic system comprises at least one primary hydraulic fluid control channel and at least one secondary hydraulic fluid control channel, the primary and secondary fluid channels being redundant and partitioned among the plurality of wheel brakes so that even if both the primary and secondary channels fail to apply pressure, braking will be lost to only a portion of the wheel brakes and the loss will be in a symmetrical pattern, and the secondary hydraulic system comprises at least one primary hydraulic fluid control channel and at least one secondary hydraulic fluid control channel. In one currently preferred embodiment, wheel braking power is provided by common fluid channels to adjacent ones of the right and left main landing gear front and aft wheel brakes to provide protection against asymmetrical wheel braking, and primary and secondary fluid channels control all four wheels of the center landing gear. In a presently preferred aspect, the wheel braking power is provided by common fluid channels to the wheel brakes of the center landing gear on an axle pair basis. Typically, the primary hydraulic system comprises three primary hydraulic fluid control channels and three secondary hydraulic fluid control channels that operate simultaneously and independently, and are arranged in redundant pairs of primary and secondary fluid channels, and each pair primary and secondary fluid channels controls the same four wheels.
In a presently preferred embodiment, the secondary hydraulic system provides pressure for alternate braking using dual, independent, closed loop analog control circuits, and the secondary hydraulic system provides dual redundant analog brake-by-wire control in the alternate braking mode for the main and center landing gears of the aircraft. The secondary hydraulic system preferably comprises a plurality of accumulators for providing an alternate supply of hydraulic power, and this alternate supply of hydraulic power is provided for an emergency braking mode in the event that both the primary hydraulic system and the secondary hydraulic system are depressurized. This alternate supply of hydraulic power is similarly provided for an ultimate braking mode providing braking pressure to a plurality of the wheel brakes, and the alternate supply of hydraulic power is also provided for a parking brake mode providing braking pressure to a plurality of the wheel brakes.
The present invention preferably provides for first and second solenoid operated shut-off valves operatively connected to the primary and secondary hydraulic systems, respectively, and to the control unit for selecting operation of one of the primary and secondary hydraulic systems, the first and second solenoid operated shut-off valves being configured to operate in a mutually exclusive manner to positively select between operation of the primary and secondary hydraulic systems without the possibility of having both systems pressurized at the same time.
In another presently preferred aspect, the control unit comprises thrust lever switches, and the solenoid operated shut-off valves are implemented through the thrust lever switches to positively prevent pressure from the primary hydraulic system and secondary hydraulic system being applied to the normal or alternate brake metering systems during take off. Typically a landing gear lever is provided controlling retraction of the landing gear, and dual redundant switches on the landing gear lever to bypass the thrust lever switches to stop the wheels during climb when the thrust levers are advanced to enable wheel braking upon retraction of the landing gear. In another preferred aspect, each of the primary brake hydraulic fluid control channels receives a software independent signal that initiates retraction braking for three seconds or until the nose landing gear is up and locked, whichever happens sooner, and an anti-skid function of the normal braking mode is inhibited during retraction braking.
The control unit in a presently preferred embodiment comprises a plurality of servo control valves controlled by corresponding dual solenoid coils for controlling the operation of the wheel brakes, respectively, and the thrust levers comprise dual thrust lever switches that break both power and ground to the first and second solenoid operated shut-off valves for the primary hydraulic system and secondary hydraulic systems when a thrust lever is advanced. Further, the control unit can additionally comprise a plurality of sensors for sensing the position of each brake pedal, such as dual redundant switches that break both power and ground to the first and second solenoid operated shut-off valves, such that depression of either brake pedal opens the first and second solenoid operated shut-off valve for the active hydraulic system. In a currently preferred aspect, the first and second solenoid operated shut-off valves also turn off hydraulic power to the servo control valves during flight. In another presently preferred aspect, the control unit further comprises sensor means for determining brake pedal application and for generating a pedal application signal indicating actuation of the wheel braking system when the brake pedal has been applied, and can also include means for sensing weight on the wheel for generating a brake inhibit signal when weight is not applied on the wheel.
The present invention provides, in a currently preferred embodiment, that the servo control valves for controlling the operation of the wheel brakes controlled by dual solenoid coils, and in a further preferred aspect, are also controlled with pressure feedback. Antiskid control is thus preferably provided on each wheel brake utilizing the dual coil servo control valves with pressure feedback. The control unit preferably comprises a pressure sensor mounted downstream of each servo control valve for detecting asymmetrical braking due to unwanted pressure applied to any wheel brake, whereby the monitor channel can select the alternate hydraulic fluid system.
These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings, which illustrate by way of example the features of the invention.